1. Field of the Invention
The present invention relates to an assembly for detection of at least one of radiation flux and contamination on an optical component. The present invention also relates to a lithographic apparatus including such an assembly. The present invention also relates to a device manufacturing method.
2. Background of the Invention
Commonly assigned, co-pending U.S. application Ser. No. 10/686,813, filed Oct. 17, 2003, discloses an electrode in the vicinity of an optical component, e.g. a mirror, in a lithographic apparatus. The electrode is biased with a positive potential relative to the optical component. Electrons generated from the optical component by a beam of EUV radiation are transported to the electrode. The current caused by the electrons is measured by a measuring device. This current is correlated to the radiation flux of the beam.
It is desirable to monitor the radiation flux at different locations and/or the contamination on the optical components in a lithographic projection apparatus. When radiation hits a surface it induces secondary electrons. The flux of the secondary electrons thus generated is related to the radiation flux on the surface and/or contamination of the surface. Presently, the secondary electrons are extracted from the surface by an electrode creating an electron attracting electric field and the current through the optical component is measured. In some instances, this measured current may not always yield the correct value for the secondary electron flux. This is caused by the following phenomenon. An photon that hits the surface induces quite an amount of secondary electrons. These electrons form a space charge layer in front of the surface of the optical component for a substantial period of time, since a large flux of secondary electrons is created and since it takes the secondary electrons some time to travel to the electrode. The space charge cloud is located between the surface and the electrode. Newly created secondary electrons are obstructed by this space charge cloud which in a way “screens” the electrode such that less secondary electrons than are generated arrive at the electrode. This may result in an inadequate assessment of the radiation flux.